Mutations - Nondisjunction
Nondisjunction is the failure of two chromosomes to separate during gamete formation, resulting in gametes with either a missing chromosome (monosomy) or an extra one (trisomy). As this animation illustrates, nondisjunction can occur either if the homologs fail to separate in meiosis I, or if the sister chromatids fail to separate in meiosis II. In either case, unless the missing/extra chromosome is a sex chromosome or a very small autosome, the condition is generally lethal. Three human trisomies, Klinefelter Syndrome, Turner Syndrome, and Down Syndrome are described in later sections.